Plant Molecular Biology 38: 1053–1060, 1998.
© 1998 Kluwer Academic Publishers. Printed in the Netherlands.
Gibberellin and abscisic acid regulate GAST1 expression at the level of
and Neil E. Olszewski
Department of Plant Biology, University of Minnesota, St. Paul, MN 55108, USA (
author for correspondence);
Current address: Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55108,
Received 27 July 1997; accepted in revised form 8 June 1998
Key words: abscisic acid (ABA), gene transcription, gibberellin (GA), nuclear run-on, RNA stability
Both gibberellic acid (GA
) and abscisic acid (ABA) regulate the expression of the GAST1 gene of tomato.
Treatment with GA
increases the abundance of GAST1 RNA while treatment with ABA blocks this effect. In
this study, the effects of GA
and ABA on the rate of transcription of the GAST1 gene and the stability of GAST1
RNA were examined. Nuclear run-on analyses detected an increase in transcription of the GAST1 gene 1 h after
treatment with transcription increasing to a maximum at 9 h after treatment. The half-life of GAST1 RNA in
-treated leaves was similar to that in control leaves. In addition, the extent of overexpression of GAST1 RNA
in transgenic tomato plants containing the CaMV 35S promoter driving the expression of the GAST1 transcribed
regionwas largelyunaffected by GA
. These results suggest that GA
stimulates the expression of the GAST1 gene
by acting only at the level of transcription. ABA treatment dramatically reduced the abundance of GAST1 RNA
in gib1 shoots through an effect at the level of transcription and did not appear to affect the stability of this RNA.
Midcourse ABA addition to the GA
-incubated shoots reversed the GA
-mediated increase in the transcription of
GAST1 gene within 15 min. Transgenic plants that either overexpressedor underexpressed GAST1 RNA exhibited
no phenotypic differences from wild type.
The interaction between two plant hormones, GA and
ABA, plays an important role in many aspects of plant
growth and development [13, 14, 20, 21]. Often the
effects of these hormones are opposite [1, 9, 12, 15].
hormones, including GA and ABA, have not been
fully elucidated at the molecular level, most of the re-
sponses are thought to be achieved through changes in
geneexpression [6, 10, 11, 14]. For example,afterger-
mination of cereal seeds, embryo-derived GA induces
the aleurone layer to synthesize and release hydrolytic
enzymes such as α-amylase and proteases that mo-
bilize macromolecules stored in the endosperm for
use by the growing seedling. GA is known to stimu-
late the transcription of genes encoding these enzymes
and ABA is able to block this increased transcription
[8, 27]. ABA also acts to destabilize GA-induced α-
amylase mRNAs . The effect of GA and ABA on
gene expression can also be opposite those observed
for α-amylase. The levels of at least 16 in vitro trans-
lation products of poly(A)
mRNA extracted from
barley aleurone layers increase after ABA treatment
and this increase is blocked by GA . The levels of
three mRNA species, alcoholdehydrogenase(ADH1),
a probable amylase and protease inhibitor, and an
unidentiﬁed barley mRNA species decrease dramati-
cally upon GA
treatment, and this GA suppression is
counteracted by ABA .
In shoots, genes regulated oppositely by both GA
and ABA are also known [22, 25]. Vainstain et al. 
showed that in ﬂowers GA-induced expression of a
gene encoding a carotenoid-associated 35 kDa protein
was antagonized by ABA.